1. Technical Field
This invention relates to the assessment of the condition of telecommunications systems while in use.
2. Related Art
Signals carried over telecommunications links can undergo considerable transformations such as digitisation, data compression, data reduction, amplification, and so on. Further distortions can be caused by electromagnetic interference from external sources
Objective processes for the purpose of measuring the quality of a signal are currently under development and are of application in prototype testing, pre-delivery testing of components, and in-service testing of installed equipment. They are most commonly used in telephony, but are also of application in other systems used for carrying speech signals, for example public-address systems.
The present applicant has a number of patents and applications relating to this technical field, most particularly European Patent 0647375, granted on 14 Oct. 1998. In this invention, two initially identical copies of a test signal are used. A first copy is transmitted over the communications system under test. The resulting signal, which may have been degraded by the system under test, is compared with a reference copy of the same signal, which has not passed through the system under test, to identify audible errors in the degraded signal. These audible errors are assessed to determine their perceptual significance—that is, errors of types which are considered significant by human listeners are given greater weight than are those which are not considered so significant. In particular, inaudible errors are perceptually irrelevant and need not be assessed.
The automated system provides an output comparable to subjective quality measures originally devised for use by human subjects. More specifically, it generates two values, YLE and YLQ, equivalent to the “Mean Opinion Scores” (MOS) for “listening effort” and “listening quality”, which would be given by a panel of human listeners when listening to the same signal. The use of an automated system allows for more consistent assessment than human assessors could achieve, and also allows the use of compressed and simplified test sequences, which give spurious results when used with human assessors because such sequences do not convey intelligible content.
Different errors may have different perceptual significance in different languages, according to the use of different phonemes, and their relative frequencies of occurrence, in each language. Multilingual test sequences are difficult to accommodate in systems using human subjects, as the human subjects would need to be familiar with all the languages represented in the test signal: otherwise spurious results could be caused by any lack of intelligibility to the listener. Automated systems do not have this drawback.
In the patent specification referred to above, an auditory transform of each signal is taken, to emulate the response of the human auditory system (ear and brain) to sound. The degraded signal is then compared with the reference signal in the perceptual domain, in which the subjective quality that would be perceived by a listener using the network is determined from parameters extracted from the transforms.
Such automated systems require a known (reference) signal to be played through a distorting system (the telephone network or other system under test) to derive a degraded signal, which is compared with an undistorted version of the reference signal. Such systems are known as “intrusive” measurement systems, because whilst the test is carried out the system under test cannot carry live (revenue-earning) traffic.
A suitable test signal is disclosed in European Patent Specification 0705501 and comprises a sequence of speech-like sounds, selected to be representative of the different types of phonetic sounds that the system under test may have to handle, presented in a predetermined sequence. The sounds are selected such that typical transitions between individual phonetic elements are represented. Typical speech comprises a sequence of utterances separated by silent periods, as the speaker pauses to breathe, or listens to the other party to the conversation. These silent periods, and the transitions between utterances and silent periods, are also modelled by the test signal.
The arrangement described above requires the use of a pre-arranged test sequence. This means it cannot be used on a live telecommunications link—that is, a link currently in use for revenue-earning traffic. This is because the test sequence would interfere with the traffic being carried and be audible to the users, and because conversely the live traffic itself (whose content cannot be predetermined) would be detected by the test equipment as distortion of the test signal.
In order to carry out tests on equipment in use, without interfering with the signals being carried by the equipment (so-called non-intrusive testing), proposals have been made to use the live speech signals themselves as the test signals. However, a problem with using live speech for this purpose is that there is no instantaneous way of obtaining, at the point of measurement, a copy of the original signal. Any means by which the original signal might be transmitted to the measurement location would be likely to be subject to similar distortions to those generated by the link under test. In test conditions it is, of course, possible to place the transmitter and receiver in close proximity, but this is impossible when testing an in-service long-distance link.
The present Applicant's International Patent applications WO96/06495 and WO96/06496 (both published on 29 Feb. 1996), and WO97/05730 (published 13 Feb. 1997) propose three possible solutions to this problem. WO96/06495 describes the analysis of certain characteristics of speech that are talker-independent, in order to determine how the signal has been modified by the telecommunications link. In WO96/06496, the content of a received signal is analysed by a speech recogniser. The results of this analysis are processed by a speech synthesiser to regenerate a speech signal having no distortions, which is compared with the original signal. WO97/05730 discloses a process in which the received signal is compared with parameters identifying speech-like and non-speech like characteristics, to derive a measurement of the quality of the signal. However, all of these techniques require considerable processing power, making it difficult to monitor signal quality in real time, and the processes all require some assumptions to be made about the incoming signal.